Zend certified PHP/Magento developer

Windows 10 unresponsive in screensaver mode – how can I figure out what process or app was taking all its CPU?

My windows 10 Dell machine is stuck on screensaver screen. I can press keyboard buttons (keyboard lights work) and I can click mouse but it seems to be processing something in the background and does not bring up the login screen.

How can I find out what process or app was taking up all its CPU at that time after a reboot?